Picture of an affected plant would help, but it is possible. Do you live in a dry part of the county? ie in the Cheshire Gap?
Mildew is associated with dryness at the roots. So the best 'cure' is to mulch very heavily when the soil is wet. This conserves the moisture in the soil and to a certain extent it stops the fungus from moving upwards from the soil. The other thing you could try is to drench the plants with a systemic fungicide, before the infection begins.

Powdery mildew doesn't always start at the base of a plant, in fact, it sometimes occurs just on the top leaves initially. If it is powdery mildew, then keeping the roots moist (as suggested in Owdboggy's answer) will help to prevent attacks, but when an attack occurs, mix up 1 part cows milk to 9 parts water and spray thoroughly with it, till run off.

The link below gives extra information on this infection, with an image of what it looks like - it does not mention the milk treatment, but it does work and is better for the environment if you use that rather than a fungicide - best with skimmed milk though, the cream in other types of milk can create a rancid smell after use.

http://www.bbc.co.uk/gardening/advice/pests_and_diseases/identifier.shtml?powdery_mildew

Funny, as we are in a very low rainfall area, the only plant which got Mildew was Monarda, so we stopped growing it. No matter what we did or where we planted it, every year, grey leaves! Phlox which is renowned for it, have none.